This invention relates to web dryers which are used in the manufacture of paper and the like and in the printing and coating of webs of paper, synthetic materials, film, etc.
Numerous types of web dryers have been developed over the years, with the dryers utilizing a variety of types of nozzle assemblies. Representative assemblies are disclosed in the above identified patents, many of which use the Coanda effect as described in detail in U.S. Pat. No. 3,549,070. This latter patent illustrates numerous assemblies having a pair of nozzles, some providing both a balanced and unbalanced Coanda characteristic when there is jet discharge into free space.
Other web dryer nozzle assemblies have been proposed which have a curved pressure plate, an upstream nozzle and a plurality of spaced downstream nozzles disposed intermediate the ends of the plate. See, for example, U.S. Pat. 3,763,571 and German Pat. No. 1,774,126.
Another type of construction is disclosed in U.S. Pat. No. 4,074,841 wherein nozzles are disposed at the upstream and downstream ends of a flat supporting plate, with the upstream nozzle creating high positive pressure in the zone between the plate and moving web. The downstream nozzle cooperates with a downstream extending diffuser sheet and functions in the manner of an airfoil to create a negative pressure in the zone between the diffuser sheet and the moving web. A spoiler, such as a hole or flange, is positioned intermediate the ends of the supporting plate.
In addition, it has been proposed in U.S. Pat. No. 3,711,960 to provide a plurality of nozzle assemblies arranged alternately on opposite sides of a moving web. In that patent, each assembly includes a single airfoil nozzle using the Coanda effect and with the nozzle assembly forming a sharp corner at the end of the support plate remote from the nozzle.
It is, of course, desirable to provide a maximum drying effect on the moving web. This cannot be accomplished if the zone between the pressure plate and web has substantially stagnant gas therein, or if the full available width of the pressure plate is not utilized. Neither can it be accomplished without minimizing the boundary layer of gas which travels along with the moving web. The effects of miscellaneous gas currents within the drying chamber on the web drying area must also be considered.
The present invention is directed to improved nozzle assemblies which are structured and cooperate with the moving web in such a manner that the above-mentioned factors are taken into account.
In accordance with various aspects of the invention, each nozzle assembly is provided with a flat pressure plate adapted to form a gas flow zone with a moving web. A primary nozzle of the airfoil Coanda type is disposed at the upstream end of the pressure plate and continuously directs gas downstream along the face of the plate. A single secondary nozzle of the impingement type is disposed at the generally right angled downstream terminus of the pressure plate to continuously direct gas initially substantially perpendicularly to the web and to gas flowing downstream along the gas flow zone. The position of the secondary nozzle assures that the full width of the pressure plate is utilized. The gas flow volume through the secondary nozzle is less than half that of the primary nozzle. Furthermore, the gas flowing from the secondary or impingement nozzle turns to take the downstream direction of the main gas flow and also serves to cause compression of the main gas flow against the web downstream of the pressure plate and nozzle assembly terminus. An increase in the rate of heat transfer to the web is thus produced in the free area between adjacent nozzle assemblies. The compressive restriction of the main gas also tends to increase its unidirectional air flow velocity, which in turn isolates the web from the effects of miscellaneous gas flow currents within the enclosed dryer housing. The overall result is a slight back pressure or increase in static pressure in the gas flow zone which assists in keeping the moving web spaced from the pressure plate, but the back pressure is insufficient to reverse the direction of main gas flow.